The present invention relates to systems and methods for generating and sustaining a corona electric discharge for use in the ignition of fuel-air mixtures, such as in internal combustion engines or gas turbines.
Generally the combustion of the fuel-air mixture in an internal combustion engine (“ICE”) or gas turbine has been initiated with a conventional spark ignition system. The spark ignition system generates an electric arc discharge in the fuel-air mixture. The arc discharge heats the immediately surrounding fuel-air mixture to an extremely high temperature causing electrons to escape from their nuclei and creating a relatively small region of highly ionized gas. The combustion reaction(s) are then commenced in this small region of ionized gas. If conditions are right, the exothermic combustion reaction(s) will heat the fuel-air mixture immediately surrounding the small region of ionized gas to cause further ionization and combustion. This chain reaction process produces first a flame kernel in the combustion chamber of the ICE or gas turbine and proceeds with a flame front moving through the combustion chamber until the fuel-air mixture is combusted.
In conventional spark ignition systems, the electric arc discharge is created when a high voltage DC electric potential is applied across two electrodes in the combustion chamber. Typically the electrodes, which in the case of an ICE are part of a spark plug, penetrate into the combustion chamber. A relatively short gap is present between the electrodes. The high voltage potential between the electrodes causes a strong electric field to develop between the electrodes. The strong electric field causes dielectric breakdown in the gas between the electrodes. The dielectric breakdown commences when seed electrons naturally present in the fuel-air gas are accelerated to a highly energetic level by the electric field. The seed electron is accelerated to such a high energy level that when it collides with another electron in the fuel-air gas, it knocks that electron free of its nucleus resulting in two lower energy level, free electrons and an ion. The two lower energy level free electrons are then in turn accelerated by the electric field to a high energy level and they, too, collide with and free other electrons in the fuel-air gas. If this chain reaction continues, the result is an electron avalanche where a large proportion of the fuel-air gas between the electrodes is ionized into charge carrying constituent particles (i.e., ions and electrons). With such a large proportion of the fuel-air gas ionized, the gas no longer has dielectric properties but acts rather as a conductor and is called a plasma. A high current passes through a thin, brilliantly lit column of the ionized fuel-air gas (i.e., the arc) from one electrode of the spark plug to the other until the charge built up in the ignition system is dissipated. Because the gas has undergone complete dielectric breakdown, when this high current flows there is a low voltage potential between the electrodes. The high current causes intense heating—up to 30,000° F.—of the fuel-air gas immediately surrounding the arc. It is this heat which sustains the ionization of the fuel-air mixture long enough to initiate combustion.
This method of initiating combustion with an electric arc discharge works reasonably well in many applications. However, there is a growing need for a different combustion initiation method which performs better than a conventional spark ignition system in demanding applications such as high air boost ICE engines and ICE engines that burn lean fuel-air mixtures. High air boost engines result in greater power output and higher efficiency. Lean fuel-air mixtures result in less pollutants discharged from the engine and higher efficiency. In these applications, the conventional spark ignition system may not perform at a level of reliability adequate to support widespread adoption.
In high air boost engines utilizing a conventional spark discharge ignition system, a greater voltage potential across the electrodes is necessary to produce the electric arc discharge because of the increased gas pressure (according to Paschen's Law). However, the maximum voltage potential at the electrodes may be limited by the dielectric strength of the insulating materials in the ignition system. If the dielectric strength of the ignition system is not sufficient, it cannot deliver the voltage potential to the electrodes to produce the arc. Even if the dielectric strength of the ignition system is sufficient to deliver the voltage potential to the electrodes and produce an arc, the increased voltage potential necessary in a high air boost engine results in greater electric energy being carried through the electrodes, an increased temperature of the electrodes, and an increased rate of electrode erosion. Electrode erosion increases the gap between the electrodes causing an even further increase in the voltage potential necessary to create an arc, and may eventually prevent the arc from being struck at all and cause misfire of the engine.
Lean fuel-air engines often operate in conjunction with high air boost levels, so conventional spark ignition systems perform problematically in many lean fuel-air engines for the same reasons explained above. Also, lean fuel-air mixtures typically require greater energy from the electric arc to effectively initiate combustion. But the maximum energy output of a conventional spark ignition system can be limited by the maximum voltage potential that can be delivered by the insulating materials in the ignition system, and can be limited by the maximum energy output possible through the electrodes before electrode erosion becomes a problem. In addition, in a lean burning engine utilizing a conventional spark ignition system, necessary heat transfer in the combustion chamber—from the electric arc to the surrounding fuel-air gas to commence combustion, and from the small flame kernel created by an electric arc to the flame front to sustain combustion—can be easily interrupted by heat loss due to turbulent gas flows, cold combustion chamber walls, etc. If the heat loss is too great, the combustion will not continue to completion.
In sum, heat is one of the drawbacks of conventional spark ignition systems, e.g., the electrode damaging heat produced by the electric arc, and the reliance upon heat transfer to initiate combustion in the flame kernel and sustain it through the flame front. Also, because of its reliance upon creating heat to ionize the fuel-air mixture, the maximum energy output of a conventional spark ignition system is limited by the amount of heat the electrodes can sustain.